Method for tuning cascade tuned circuits



July 18, 1950 E. o. KEIZER 2,516,056

METHOD FOR TUNING CASCADE TUNED CIRCUITS Filed Feb. 26, 1946 Eqgezze 0. ez'zer of tuning cascade 'tu-ned Patented July 18, 1950 2,516,056.. T TORTUNING cAsoA E TUNED" L1ciaoUrrs.

Eugene: Keite'm Brincetom. N: J 3,; assignor. to i Bladiozjfioriporationrof America-,5, corporation of e a r 1 e p icauouireuruery-ze, 194e, s'eriarhotetmsrrr This invention relates rc'uits ahd to suitableapparatus which may be used to carry out 'the method. p v

Ordinarily, the aligning for tuning-caravan a few cascade tuned circuitstothe same frequency requires many hours (ii-painstaking work; Where several hundred such circuits-are-involved?asin certain types of ariii ficial delay lines, it would be almost an impossible task to properly tune all of them withoutsome jspe'cial-,method as offered by the present invention.

One object of the present invention isto pro Vide a method of tuning a plurality o f'c'a'scade' requiring only i simple: adjusting step qtor teach;

circuit.

These and other objects will be more apparent from the description of the invention which follows together with a study of the drawings of which:

Fig. 1 is a side view of a number of cascade circuits together with a schematic diagram of apparatus suitable for carrying out the tuning method.

Fig. 2 is a diagram similar to that of Figure 1 but depicting the cascade circuits in a schematic manner.

Apparatus which may be employed in carrying out the method of the invention will now be described. Referring to Figure 1, a high frequency oscillator l supplies a suitable signal. Metal rods R1, R2, Ra, R4, R5, R6, and R1 comprise cascade circuits which are to be tuned to the frequency of the oscillator. These rods project inside a metal cabinet 3 and the lengths of the rods extending inside the cabinet are adjustable by means of turning them on the threads 4 engaging the bushings 5. To the first of these rods R1 there is capacitively coupled a detector circuit. This circuit comprises a detector crystal 6, by-pass condenser I, and a registering milliammeter 8.

The method is carried out by first widely detuning all the cascade circuits. Next, the oscillator is set at some desired high frequency as for example 1,400 megacycles. The first circuit of improvedmethod the caseade isthen tuned tdmaximum-rescnance by adgasting the rtgth-pf-Itid R1 extending-inside. the eabih et. Ih'ecui rent fiowing-in the detec'tcn circuit will be a rnaximuIn when the desirei cornditilon haszbe'eii reached The mea-imumvaiueis indicated tyrtne r'eadirrg on the milliamine't The next-circa iistheni tuneduntilthe m lliamrhe'te-r on thie -first :circuitmegis'ters 'a' minimum: Thiss is lfilrew' e=-aeconrp shed-by adjustin'g t elengithl'ofwrod; Re e'xterrcli'n'ginside: the metal bOXu thirdic'ircuit is tuned: until: th'e mfllramrne terlagai-n registers-a maximum, and so Iona-each oddi numlarered circuit:beingatuned to a max'ir nmnand each even numbered ci cuitbeihg: tuned t'o al-ininimum untillaillare 'aligne Circuits of the type-illustrated:haveraivery -highr unloadeiil Q whichrmay: rangetfrom. 5.00am 2,090. For this reasoi their'rtuningrby metho'dszordi narilytemployede lextremelyrd'ifliculti and time censu-ming::., Each set the mods ;.is analogous to fa cascadetjcireuit. having capacitance and linduc't ancesaslillustratedl innEigur'e-lZ, oflwhich the res chant wavelength is. approximately rourstimes athe lengthen the rod; unless .thelatter;isshunted by: extra 5 capacity at; the .zcp'enza Thus; .by Marysing: the :ien'gth Qfiillhefl'adsttha resonant; wave-e: length may be varied.

The relatively high currents present near the end of each rod closest to the aperture through which it enters the metal box results in relatively high inductive coupling between circuits at this end. At the other end of the rod, there is relatively little current, hence little inductive coupling between circuits, but the capacitive coupling is relatively high. The coupling consists of the net eifect of capacitive and inductive coupling between adjacent tuned rods. These two efiects oppose each other, but ordinarily the inductive coupling predominates.

If all the rods were very precisely machined so as to be perfectly uniform in thickness and if the metal box, itself, did not introduce varying efiects,

the circuits could be tuned by screwing the rods into the box until each one had been lined up against a straight edge placed against all the ends. However, there are always small variations in the dimensions of the rods and small but important differences due to inaccuracies in mounting are practically impossible to keep out. For this reason, it is necessary to tune the rods either by varying their lengths or by varying the capacities of their open ends to ground.

Other expedients may be used to aid in tuning circuits such as those illustrated. In place of the crystal detector, a vacuum tube detector circuit may be employed. Shields may also be introduced between the rods to vary the capacitive coupling. But, regardless of how the details of the apparatus are varied, the method of tuning still remains as described.

The series or cascade circuits illustrated constitutes a band-pass filter. In tuning the circuits, the oscillator is set to give a signal of the pass band center frequency and this signal is fed to the first tuned circuit of the cascade. The first circuit and all succeeding circuits are then tuned as has been described.

The range of frequencies which may be used in the present method is quite wide. The method is applicable to any frequency from the broadcast band upwards.

justing a source of high frequency oscillations to said desired frequency, tuning the first .circuit in said series to maximum resonance with said oscillations, tuning the second circuit to a point such that the resonance of the first circuit is a minimum, tuning the third circuit such that the resonance of the first-circuit has returned to a maximum and continuing the process throughout the remaining circuits, tuning each even numbered circuit such that the output of said first circuit is a minimum andtuning each odd numbered circuit such that the output of said first circuit is a maximum.

2. A method according to claim 1 in which said cascade circuits have a high Q.

3. A method according to claiml in which sai cascade circuits comprise metal rods.

4., A method of tuning a series of cascade circuits in order to provide a band-pass filter which will pass a predetermined band of .high frequencies, said method comprising adjusting a source of high frequency oscillations to the pass band center frequency, and successively tuning each circuit such that each odd numbered circuit is tuned to the point of maximum output in the It has most application, however, 7 in very high frequency Work and gives best re 4 first circuit and each even numbered circuit is tuned to the point of minimum output in the first circuit.

5. A method according to claim 4 in which said cascade circuits have a high Q.

6. A method according to claim 4 in which said cascade circuits consistof metal rods.

7. A method of tuning a series of cascade circuits to a desired high frequency, comprising widely detuning all of said circuits, adjusting a source of high frequency oscillations to said desired frequency, and successively tuning each circuit such that each odd numbered circuit is tuned-to the-point of. maximum resonance in the first circuit and each even numbered circuit is tuned to the point of minimum resonance in the REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS.

Number Name Date 2,130,824 Jones Sept. 20, 1938 2,138,209 Seeley a Nov. 29, 1938 2,262,707 Farrington Nov. 11, 1941 2,410,656 Herold Nov. 5, 1946 H OTHER REFERENCES The Radio. Amateurs Handbook, published by the American Radio League, West Hartford 7, Conn., published in November 1945. 

